1. Technical Field
The present invention generally relates to ophthalmic devices such as contact lenses having a high water content with improved evaporative dehydration.
2. Description of the Related Art
Soft contact lenses have been available since the 1980s. It is important that contact lenses be comfortable and safe to wear. However, while there are many people who can successfully wear contact lenses, there are a number of people who can wear contact lenses for only a short period of time due to, for example, contact lens related dry eye. Symptoms of this disorder include, for example, thin and/or unstable tear films, corneal staining and subjective symptoms such as ocular discomfort, burning/stinging and dryness. Contact lens wear may trigger the onset of these symptoms or may exacerbate the symptoms.
Although lenses with high water contents are softer, more lubricious and more comfortable to wear, such lenses may not have one or more properties useful to provide comfortable and safe wearing of the contact lenses. For example, a particular problem associated with high water content contact lenses is evaporative corneal dehydration. As free water in the lens is lost due to evaporation, it is replaced with water from the cornea. Evaporative water loss at the anterior lens surface is a potential cause of contact lens dehydration and of post-lens tear-film depletion, which in turn, may lead to discomfort, dry eye, corneal staining and/or other damage to the eye. Dehydration of the epithelium results in corneal damage and therefore corneal staining. This staining is usually limited to the superficial 2 to 3 layers of the epithelium and spread over the inferior portion of the cornea, but if the stimulus is sufficiently great, damage can be deep and severe allowing rapid diffusion of fluorescein into the stroma. The staining can occur rapidly within a few hours of lens insertion but can take 4 to 6 hours or more.
Historically, high water content contact lenses were typically made by lathing the high water content lens from a cylindrical blank of polymerized lens material (cylindrical blanks are commonly referred to as a “button”). For example, the high water content contact lenses are made by (a) polymerizing the initial monomeric mixture in tubes to provide rod-shaped articles, (b) cutting the rods into buttons, and (c) lathing the buttons into contact lenses. However, a surface of a lens produced from lathed buttons can be quite different from the surface of a lens cast from molds.
U.S. Pat. No. 6,245,830 discloses a high water content and high water balance contact lens made of a homopolymer or copolymer having more than 80 mole percent and preferably more than 90 mol percent of 2,3-dihydroxypropyl methacrylate (GMA) and up to 20 mol percent, generally from 0.05 up to 10 mol percent of a reactive pyrrolidone such as N-vinyl pyrrolidone.
U.S. Pat. No. 6,627,674 (“the '674 patent”) discloses a high water content and high water balance contact lens made of a homopolymer or copolymer having more than 80 mole percent and preferably more than 90 mol percent of GMA and having an equilibrium water content of at least about 60 percent by weight and the contact lens has a water balance of more than about 8 relative to poly(2-hydroxyethyl methacrylate). The '674 patent further discloses that GMA polymers can be polymerized in the presence of a reactive polar aprotic diluent, such as N-vinyl pyrrolidone and/or a non-reactive polar aprotic diluent.
There remains a need for a high water content contact lens possessing superior dimensional stability and having a low rate of dehydration such that evaporative corneal dehydration can be reduced.